Dear Christ, I know we said we wanted more topial threads and not endless 'Halo FTW' but this is getting ridiculous.
As to your question, it is impossible for us to determine when existence ends, thus we should't cncern ourselves with it. Since human life is about (paradoxically) the absence of an end, what we sould be more worried about is the moment in which we are not ending. (I say that human life is an absence of an end because as long as life continues it has not ended and once it ends there is no life in which to not end.)
On a larger scale, it is probably safe to assume that humanity will come to an end long before the end of the universe, whatever Doctor Who would have you believe, and so once more it should not be a concern. Humans are fascinating in this regard that we consistently look beyond the now, and only to the point yet to appear, meaning that (since the point gone has already left, and the point yet to appea has not arrived), we always miss out on te only moment in which we are living. Existence is a series of moments, and while the moments are interconnected, the moment itself is only around for the briefest of measurs of time before it is gone, and refusing to live in it seems rather pointless. Yes one should (in our modern society) plan for the moments yet to come, but notat the expense of the moments in which the moments yet to come arrive.
I'm being overly generalised. If I plan for a party, I should enjoy the party when it happens, this seems a fairly sensible deduction yes? Imagine that your life is one massive party, to which you have a constant invite, yet because you insist on looking at when the party finishes, you miss the entire party. The end is not the point of the party, the party is the point of the party. Don't miss it by worrying about what happens when the check finally comes around.
We will never know when existence ends, either personally or for us all, but we will know the moments we took to enjoy the moment we lived in, rather than the moment we looked toward, that somehow never arrived.